In chemistry, there is a rule called "the octet rule", which says that the outermost energy level of atoms is completed when it reaches 8 electrons.
In this case, oxygen is no exception, this atom has 6 electrons in its last energy level and requires 2 more to complete its octet. In the following image, we see the lewis structure for oxygen where each dot represents an electron. Each individual electron represents a space where another electron can join to form a bond and complete the octet.
Finally, to answer the question, hydrogen atoms have only one electron, so we need 2 hydrogen atoms (one for each individual electron of oxygen) to complete the octet as we see in the next image. Therefore, the answer is C (2)